Display devices traditionally operate by refreshing the displayed image a number of times per second, such as sixty times per second. When power is removed from such a display device, the display device no longer displays an image. Examples of these types of display devices include cathode-ray tube (CRT) display devices, as well as flat-panel display devices that employ plasma, light-emitting diode (LED), and/or some types of liquid crystal display (LCD) technologies.
More recently a type of display device has been introduced in which an image remains displayed even when power is removed from the display device. Power is utilized in such so-called bistable or multistable display devices primarily when the state of the image being displayed has to be changed. This type of display device uses less power than conventional display devices. Currently, this type of display device is most commonly used for electronic book reader (“e-reader”) devices available from a number of different manufacturers.
Displays can be broadly categorized as either emissive or reflective. Emissive display devises have their own light sources, which emit light and thus can be used even in low ambient light conditions. Reflective display devices do not require their own light sources, but instead rely on the modulated reflection of ambient light, although in some cases reflective displays are provided with an auxiliary light source for viewing in low ambient light conditions. Reflective display devices typically use less power than emissive display devices because they do not need to provide their own light and because they are often multistable; that is, the image remains even when power is removed from such a display device. Although emissive display devices are more easily viewed in low ambient light conditions, reflective displays provide better viewing in bright outdoor lighting where stray reflections often overwhelm the relatively weak light source of emissive displays.